Most failures of power transistors in electronic circuits are due to overheating caused by excessive current. Thus, the incorporation of temperature sensing improves the overall reliability of a power electronics system. If a power transistor starts operating at an excessive temperature, a temperature sensor detects that excessive temperature, and sends a signal to a control circuit. The control circuit then turns off the power transistor or decreases its current load. However, in order to be effective, such temperature sensors must be accurate and quick to respond to changes in temperature. Otherwise, the power transistor could fail before the excess temperature is detected and corrective action taken.
Crude mechanical thermostats are sometimes attached to the heat sink of a power transistor in order to sense temperature. However, such mechanical thermostats are not reliable and do not easily interface with control circuits.
Electronic temperature sensors are often glued or attached to the heat sink or package of the power transistor. Such external temperatures sensors are not in close proximity to the power transistor, and thus there is a significant delay between the temperature rise in the power transistor and the sensors detection of that temperature rise. As a result, power transistors might fail before the excessive temperature is detected.
Sometimes temperature sensors are inserted inside the power transistor package. However, most inserted temperature sensors require at least three external leads for power and control signals. Therefore, a package with at least six leads must be used to enclose the power transistor with three leads and the temperature sensor with at least three leads. However, although three lead and five lead packages are standard packages, hence inexpensive, six lead packages are not standard packages and must be special ordered at a higher cost. In addition to the packaging problem, there is still a significant delay between the rise of temperature in the power transistor and the sensor's detection of that temperature rise.
Another type of temperature sensor called a PTAT (Proportional To the Absolute Temperature) is sometimes monolithically integrated with the power device. PTATs are temperature sensitive semiconductor circuits that generate a voltage or current that is proportional to the absolute temperature. Because of this relationship, fewer external components are needed in the control circuits to sense temperature, and the control circuits can more easily determine what the power transistors actual temperature is. Therefore, the control circuits can better determine the appropriate corrective action to follow in order to protect the power transistor. Because they are monolithically integrated with the power transistor, PTATs are quicker to detect changes in the power transistor temperature. In addition, they only require two leads out of the power transistor package. However, PTATs are not capable of detecting temperature in high voltage power transistors. If the power transistor handles more than 20 volts, the PTAT suffers from voltage breakdown and ceases functioning properly. Therefore, PTATs cannot be used to detect temperature in most power transistors. PTATs are described in greater detail in Soclof, S., "Applications of Analog Integrated Circuits", Prentice Hall (1985) pp. 273-284.
Power MOSFETs have an on-resistance that varies linearly with temperature. Copending Application Serial No. 223,059, filed July 22, 1988 and titled HIGH POWER MOS TRANSISTOR WITH VOLTAGE, TEMPERATURE, CURRENT, AND POWER SENSING CAPABILITY discloses a technique for monitoring device temperature using current mirrors. However, this technique is not effective with IGBTs which do not exhibit similar linear behavior with respect to temperature.